Method of amplifying an optical signal, an optical amplifier for performing the method, and use of such an optical amplifier as a source of light

ABSTRACT

An optical amplifier for signal light. The amplifier amplifies signals at a certain signal wavelength and consists of an active optical waveguide (4) of simple wave type geometry at the signal wavelength, a pump laser (2) for obtaining inverted population between the energy levels involved and coupling devices for coupling pump light and signal light in the active optical waveguide (4). The active optical waveguide is doped with ions of transition metals or metals from the groups of Lanthanides or Actinides. This doping is in a cylinder shell-shaped area (13) around the axis of symmetry of the waveguide (4) so that the reduction in the optical power originating from amplified spontaneous emission is greater than the reduction in the optical power originating from amplified signal light.

BACKGROUND OF THE INVENTION

The invention relates to an optical amplifier which amplifies signals ata certain signal wavelength and consists of an active optical wave guideof simple mode geometry at the signal wavelength, and a light source,e.g. a pump laser for obtaining inverted population between the energylevels involved and coupling devices for coupling pump light and signallight in the active optical wave guide. The wave guide is doped withactive ions which are substantially positioned in a cylindricalshell-shaped area around the axis of symmetry of the wave guide.

The use of optical amplifiers constitutes a technically interestingfield, because optical power can be amplified with these directly in anoptical fiber or wave guide without conversion of the optical energy toelectric energy.

Such optical amplifiers are used or may possibly be used in fiberoptical communications systems, for fiber lasers, for fiber opticalsensor systems or the like. Also amplifiers manufactured in plane waveguide technology can be used within the above-mentioned fields.

Optical amplifiers and in particular fiber optical amplifiers operate bythe process "stimulated emission", where a material can emit light withthe same wave vector and phase as incoming light, it beingenergy-supplied (pumped) with light from a strong pump source, typicallya laser having a lower wavelength than the signal. Amplification in amaterial capable of amplifying by stimulated emission depends on thedipole moment of the quantum transition considered, on the wavelength ofthe signal and on the size of the relative population of the two quantumstates directly involved. Where the upper one of the two levels has agreater population than the lower one, so-called inverted population, itis possible to obtain amplification. This inverted population isgenerated via absorption of pump light.

The materials capable of forming the basis for stimulated emission cangenerally be divided into two groups: 3-level systems and 4-levelsystems.

The European patent application 368 196 discloses a fiber amplifier inwhich the active Er³⁺ ions are distributed along the axis of symmetry ofthe fiber in a cylinder shell which is contained completely within thecore region. The optical fiber amplifier is pumped with energydistributed on several transversal wave types, and the cylinder shell istherefore spaced from the axis of symmetry corresponding to the distancewhere the pump light has maximum intensity. This design is not optimalfor amplification of light by means of 4-level systems with amplifiedspontaneous emission (ASE) at lower wavelengths than the signal.

The invention also concerns a method of amplifying an optical signalwith an optical amplifier, which is stated in claim 1. Hereby it ispossible e.g. to obtain discrimination between the formation ofspontaneous photons at a shorter wavelength (e.g. 1050 nm) by formationof signal photons at signal wavelength (e.g. 1340 nm) by stimulatedemission. Amplification of signal photons will be increased considerablyhereby, because the pump light creating inverted population for thelevels involved is utilized better for the 1300 nm transition than forthe 1050 nm transition. The invention can e.g. be worked in connectionwith effective amplifiers based on Nd³⁺ doped ZBLAN glasses, but can beutilized by any 4-level optical amplifier to which it applies that thesignal wavelength is greater than the wavelength of the dominatingamplified spontaneous emission.

One of the crucial problems of e.g. Nd³⁺ doped fibres is thus that alarge amount of ASE is formed at considerably lower wavelengths than thewavelength of the signal to be amplified. The invention reduces thisproblem by using a special design of the active fiber. When the fiber ismanufactured such that doping is in a ring around the axis of symmetry,the formation of the ASE having a low wavelength will be reducedrelatively more than the stimulated amplification of signal light, andamplification at the signal wavelength will therefore be increasedconsiderably. Removal of the doping from the center of the core reducesthe amplification of the light both at the short and at the longwavelength, but the decisive point is that amplification of the lighthaving the short wavelength is reduced relatively more than thelong-waved light.

Since the undesirable ASE has a considerably lower wavelength than thesignal, the region occupied by this ASE in the fiber, e.g. described bythe dot size D_(ASE), will be considerably smaller than the dot sized_(S) of the signal. Amplification of the short-waved ASE thereforediminishes more rapidly than the amplification of the signal if thedoping of the active ions is moved away from the center of the waveguide and is positioned in a cylinder shell-shaped area concentric withthe axis of symmetry of the wave guide. The invention utilizes thiscircumstance for suppressing the undesirable ASE to achieve greateramplification of the desired signal. Another object of the invention isto provide an optical amplifier having improved efficiency over theprior art.

This object is achieved by an optical amplifier having the featuresdefined in the characterizing portion of claim 2. A considerably part ofthe pump energy is hereby absorbed in parts of the active optical waveGuide, where the proportion between formation of spontaneous emission atconsiderably shorter wavelengths than the signal wavelength and theformation of spontaneous emission around the signal wavelength isgreatest. Thus, also the proportion between amplification of thespontaneous emission at considerably shorter wavelengths than the signalwavelength and amplification of the signal light is greatest.

The active optical wave guide is doped with ions of transition metals ormetals from the groups of Lanthanides or Actinides.

Claim 3 defines the active optical amplifier having a wave guide which,according to the invention, is to be produced with the doping which doesnot coincide with the core, if the active ion is of the 4-level type.Improved amplification will be obtained by moving the doping away orpartly away from the core. If the inner radius of the doping is calleda₁ and the outer radius of the doping is called a₂, these diameters willbe related to the core radius a in the following manner:

    a<a.sub.2 a.sub.1 <a.sub.2.

Claim 4 defines an amplifier consisting of three main elements: theactive wave guide, coupling devices for coupling pump light and signallight as well as one or more pump sources. In case of Nd³⁺ doping thepump source is to have a wavelength around 0.795 μm, while other activeions demand other pump wavelengths. The pump wavelength will be smallerthan the signal wavelength under normal conditions.

Claim 5 defines an amplifier having an active wave guide doped with twoor more active ions having overlapping amplification regions. This maybe an advantage when using ions which themselves cannot cover the entireinteresting amplification region.

Claim 6 defines an amplifier having two amplification regions, e.g.coinciding with the most interesting transmission fields within fiberoptical communication. Such an amplifier may be produced using twodifferent active ions having different amplification regions, e.g. Nd³⁺for the 1300 nm region and Er³⁺ for the 1550 nm region.

Claim 7 defines an embodiment of an amplifier for the situation wheresuch a double ion doping is necessary to obtain amplification in twowavelength regions. Here doping is performed with a 4-level ion in acylinder shell-shaped area and with a 3-level ion in the core area,which is done to obtain the greatest amplification.

Claim 8 defines preferred ion types, which have been found to be moreeffective than others. Ions operating according to 4-level systems arepreferred, because active optical wave guides made with doping of4-level ions do not exhibit ground state absorption. The ions Nd³⁺ andPr³⁺ are of special interest for the region around 1300 nm, which haveboth exhibited good amplification properties in this region.

Claim 9 shows that the host glass for the production of the active waveguide is of importance for the amplification properties, because thehost glass affects the energy of the quantum states of the active ions.The host glass thus influences absorption wavelengths and emissionwavelengths. Host glass based on SiO₂ is of special importance, becauseordinary transmission fibers for optical communication are made of thismaterial.

Claim 10 describes host glasses made on the basis of metal fluorideglasses, e.g. ZBLAN, which have another influence on the absorption andemission of the active ions than SiO₂ based glasses. ZBLAN glasses areparticularly interesting, because it has been found possible to makeNd³⁺ doped fibres having considerable amplification around 1300 nm inthis material.

Claim 11 defines a preferred embodiment of the active optical waveguide. One of the most interesting uses of optical amplifiers is inconnection with fiber optical transmission systems. In this case it isextremely advantageous that the active wave guide is an optical fiber.Then the active ions will lie like a cylinder shell concentricallyaround the axis of symmetry of the fiber and along the fiber core. Theactive doping can either overlap the core or be positioned right out inthe cladding of the fiber.

Claim 12 defines an amplifier having an active wave guide based on planewave guide technology. For certain uses, and especially if aparticularly inexpensive amplifier component is desired, it isadvantageous to make the amplifier as an integrated optical component.Then the wave guides, and thereby also the active optical wave guide,will have to be made in plane wave guide technology with wave guideshaving a substantially rectangular cross-section. Examples of substratematerials for such integrated components are Si/SiO₂, LiNbO₃ and III-Vsemiconductor materials.

Claim 13 defines the wave type of the pump light. If the pump lightpasses through the active wave guide in the fundamental transversal wavetype, advantages are obtained in particular for 3-level ions with activedoping in the core, but also 4-level ions having cylinder-symmetricaldoping can be excited. It is important in this case that the 4-levelions are doped in a cylinder shell for the greatest amplification to beachieved.

Claim 14 defines the use of pump light with several wave types. For4-level ions even greater amplification can be obtained if the pumplight passes the active wave guide in several transversal wave types.The reason is that the higher order wave types give rise to a greaterintensity of pump light away from the axis of the wave guide and thushave better overlap with the cylinder shell-shaped doping.

Claim 15 describes a preferred structure of the active optical fiber.Calculations show that an Nd³⁺ doped ZBLAN fiber can be optimized tobeing particularly amplifying if doping is arranged in a region definedby:

    a.sub.1 /a is in the range [0.6; 1.4]

    a.sub.2 /a is in the range [0.85; 4.0]

Claim 16 defines the use of the amplifier described in claims 2-15 as alight source. When reflection devices, such as mirrors, couplers,reflection screens, etc., are arranged at one or both ends of the activewave guide, part of the light emitted from the amplifier will bereturned to the amplifier and amplified additionally. Using such areflection device broad spectrum optical sources having a shortcoherence length can be produced. Using reflection devices at both endsof the active wave guide, a laser can be produced, the laser cavitybeing defined by these two reflection devices.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

The invention will be explained more fully below in connection withworking examples and with reference to the drawing, in which

FIG. 1 schematically illustrates a configuration of an opticalamplifier,

FIG. 2 shows the radial intensity distribution of the signal, theamplified spontaneous emission and the pump light together with therefractive index profile and the doping profile for an embodiment of anoptical amplifier according to the invention,

FIG. 3 shows an example of the relation between the amplification in thefiber and the inner and outer doping radius relatively to the coreradius,

FIG. 4 shows the theoretically achievable amplification with doping inthe core and with optimum positioning of the doping in a cylinder shellalong the wave guide, respectively, using various wave types,

FIG. 5, at the top, shows the optimum selection of inner and outerdoping radius relatively to the core radius as a function of thenumerical aperture of the fiber and three pump power levels, and thecorresponding amplification is shown at the bottom,

FIG. 6 schematically shows the position of the active ions in apreferred embodiment of the optical amplifier according to theinvention,

FIG. 7 shows an alternative embodiment of an optical amplifier accordingto the invention and realized in plane wave technology, and

FIG. 8 shows a use according to the invention where the amplifier of theinvention is used as a laser.

DETAILED DESCRIPTION

FIG. 1 illustrates a typical configuration of an optical amplifier. Theweak signal to be amplified is passed to the amplifier via a waveguide 1. The amplifier is pumped by a pump source with wavelengthscoinciding with one or more of the absorption transitions of the activewave guide. The pump light and the signal light are coupled into anactive wave guide 4 by means of a coupling device 3. The amplifiedsignal light is passed further on from the active wave guide to a waveguide 5. These are general principles of all optical amplifiers.

FIG. 2 illustrates the invention with an example concerning an Nd³⁺doped ZBLAN fiber. The figure shows the radial intensity distribution ofa wave guide signal at 1.34 μm in a dashed line 6, the amplifiedspontaneous emission at 1.05 μm in a broken line 7, and pump light at0.795 μm as a solid line 8. These radial intensity distributions areshown as normalized power distributions. The figure moreover shows therefractive index profile 10, which is a step index profile in thisexample indicated by solid lines, and the doping profile 11, which is acylindrical shell-shaped step index profile indicated by dashed lines.

FIG. 3 shows an example of the result of a theoretical calculation ofthe amplification in a fiber corresponding to the one shown in FIG. 2.Amplification is indicated as a function of an inner relative dopingradius a₁ /a and an outer relative doping radius a₂ /a for a pump powerof 200 mW and a signal power of 0.1 μW, a numerical aperture of 0.3, acore radius of 1.02 μm and a fiber length of 5 m.

FIG. 4 shows, for the same example of calculation, the achievableamplification respectively with doping in the core (solid line) and withoptimum position of the doping according to the invention (dashed lines)as a function of the pump power. The signal power is 0.1 μW, the coreradius 1.02 μm, the fiber length 5 m, and the numerical aperture is 0.3.The amplification for two different pump wave types is shown by long-and short-dashed curves, respectively.

For the calculation example FIG. 5 at the top shows the optimumselection of inner and outer relative doping radii as a function of thenumerical aperture for the fiber and three pump power levels indicatedby a solid line (200 mW), short-dashed line (100 mW) and a long-dashedline (50 mW), respectively. The signal power is 0.1 μW, the fiber lengthis 5 m, and the cut-off wavelength for wave type LP₁₁ is 800 nm. Thecorresponding amplification is shown at the bottom. It will be seen inthe shown example that the doping material is positioned within acylinder shell whose inner radius a₁ corresponds to 1.4a, a being theradius of the fiber core. The outer radius a₂ will be about 2.5a.Preferably, a₁ /a and a₂ /a will be in the ranges [0.6; 1.4] and [0.85;4.0].

FIG. 6 shows the embodiment which is described in claim 6. The opticalfiber 12 is seen in cross-section. The ring-shaped doping with 4-levelions is shown as 13, while the central doping with 3-level ions is shownas 15. In this case the core area 14 coincides with the doping of the3-level ions. Graphical representations of the refractive index profileas well as the two active dopings transversely to the wave guide areshown at the bottom.

FIG. 7 shows the embodiment based on the use of plane wave technology.An active wave guide 17 is placed on a plane substrate 16 which may beeither crystalline or amorphous. The wave guide is doped with activeions, so that it can be used in an optical amplifier according to theinvention.

FIG. 8 shows a structure of an optical amplifier corresponding to theone shown in FIG. 1, but with partially transparent reflection devices20, 21, such as mirrors, positioned at the ends of the active wave guide4. A laser cavity will hereby be formed between the mirrors 20, 21.

Considerably more complicated configurations with several couplingdevices, several pump sources, several active wave guides and othercomponents, such as optical band-pass filters and optical isolators, maybe used in special cases.

Generally, a 3-level system just involves three quantum states in theprocess because the ground state of the system operates as the loweststate for the amplifying quantum transition. These systems thereforehave the less attractive property that they absorb signal light if theyare not pumped. An example of a 3-level system is Er³⁺ with emissionaround 1550 nm.

A 4-level system involves four or more quantum states in theamplificaton process, because the ground state is not the lowest statefor the amplifying quantum transition. 4-level systems therefore do notexhibit signal absorption from the ground state. An example of a 4-levelsystem is Nd³⁺ with emission around 1340 nm.

Particularly optical communication in the wavelength regions around 1300nm and 1550 nm is of interest. For uses around 1550 nm, Er³⁺ dopedfibres are employed for fiber based amplifiers, while amplifiers for1300 nm have been long in coming. So far, the Nd³⁺ doped fiber amplifierhave been the most examined amplifier for the 1300 nm region. However,this amplifier has two serious problems. One is the excitation of ionsfrom upper laser level to higher energy levels at absorption of signallight (excited state absorption), another is the generation ofspontaneous photons around 1050 nm which are amplified vigorouslythrough the fiber at the expense of the signal amplification around 1300nm.

The problem of excited state absorption can be solved by using anotherhost glass material than the commonly used SiO₂, e.g. ZBLAN.

With respect to the amplified spontaneous emission the only way ofreducing this has so far been to use one or more filters positionedalong the fiber.

An exact numerical model of a fiber amplifier has been used forevaluating various structures of the active optical wave guide. Theresult is that in a large number of cases the active doping should becompletely outside the core of the fiber and should in certain cases beoutside and within the core. In all the considered cases it was possibleto obtain greater amplification using the cylinder shell-shaped dopingaccording to the invention.

The invention and its results are exemplified by the followingtheoretical examinations: An Nd³⁺ doped ZBLAN fiber is considered. Ithas an ND³⁺ concentration of 9.5×10²⁴ m⁻³ a numerical aperture of 0.3 acore radius of 1.02 μm, a length of 5 m and a step index profile. Thesignal wavelength is 1.34 μm, and the wavelength of the pump light is0.795 μm. For a pump power of 200 mW coupled into the active fibertogether with a signal power of 0.1 μW, amplification can be increasedfrom 6.7 dB for doping in the core to 13.3 dB for doping positionedbetween an inner radius relatively to the core radius of 1.4 to an outerrelative radius of 0.3.

What is claimed is:
 1. A method of amplifying an optical signal with anoptical amplifier having a 4-level lasing system, said optical signalhaving a wavelength-dependent field distribution transverse to anoptical wave guide (4) doped with active, light-emittable ionspositioned in a cylindrical shell (13) coaxially with the axis ofsymmetry of the wave guide (4), in which cylindrical shell (13) theactive ions are excited for stimulated emission of optical energy at afirst wavelength corresponding to the signal by pumping of opticalenergy at one or more other wavelengths, said method comprisingproviding the cylindrical shell (13) with active ions at such a distancefrom the axis of symmetry of the wave guide (4) that the active ions areexcited for radiation of spontaneous emission in an area of the waveguide (4) where the reduction in the optical power originating fromamplified spontaneous emission is grater than the reduction in theoptical power originating from amplified signal light, the wavelength ofthe optical signal being greater than the wavelength at the dominatingamplified spontaneous emission.
 2. An optical amplifier for an opticalsignal having a field distribution depending on the structure of waveguides and the wavelength of the signal, comprising an active, opticalwave guide (4) doped with active and light-emittable ions in a 4-levellasing system so that the optical signal with a first wavelength isamplified by passing the active, optical wave guide (4) when said waveguide is pumped with optical energy at one or more other wavelengths,said active ions being substantially positioned in a concentricalcylindrical shell (13) around an axis of symmetry of the wave guide (4),in which cylindrical shell (13) the optical pump energy excites theactive ions for stimulated emission of optical energy at said firstwavelength, said cylindrical shell (13) with active ions beingpositioned at such a distance from the axis of symmetry of the waveguide (4) that the reduction in the optical power originating fromamplified spontaneous emission is greater than the reduction in theoptical power originating from amplified signal light, the wavelength ofthe optical signal being greater than the wavelength of the dominatingamplified spontaneous emission.
 3. An optical amplifier according toclaim 2, wherein the active wave guide is of the step index type with acore radius a, the active ions being present in said cylindrical shell(13) between an inner radius a₁ and an outer radius a₂, the radii a, a₁and a₂ having the relationship, a₁ <a₂ and a<a₂.
 4. An optical amplifieraccording to claim 2, including a coupling device (3) for coupling pumplight and signal light, as well as one or more pump sources (2) emittinglight at wavelengths different from the first wavelength of the signallight.
 5. An optical amplifier according to claim 2, wherein the activeoptical wave guide (4) is doped with two or more active ions withoverlapping amplification wavelength regions.
 6. An optical amplifieraccording to claim 2, wherein the active optical wave guide (4) is dopedwith two or more active ions with different amplification wavelengthregions.
 7. An optical amplifier according to claim 2, wherein a firstactive 4-level ion is doped in said cylindrical shell (13), while asecond active 3-level ion is doped in the core area (15).
 8. An opticalamplifier according to claim 2, wherein said active optical wave guide(4) is doped with Nd³⁺ and Pr³⁺ ions .
 9. An optical amplifier accordingto claim 2, characterized in that the active optical wave guide (4) ismade of a host glass, mainly consisting of SiO₂.
 10. An opticalamplifier according to claim 2, wherein the active optical wave guide(4) of the amplifier is made of a fluoride host glass, preferably of thetype ZBLAN.
 11. An optical amplifier according to claim 2, wherein theactive optical wave guide (4) is an optical fiber with a circularcross-section and a core area (14) which is positioned coaxially withthe longitudinal axis of the wave guide (4).
 12. An optical amplifieraccording to claim 2, wherein the active optical wave guide (17) is madeon a plane substrate (16) with a substantially rectangularcross-section.
 13. An optical amplifier according to claim 2, whereinthe amplifier is pumped with pump light which is transmitted in theactive optical wave guide (4) with a wave length of the lowest order.14. An optical amplifier according to claim 2, wherein the amplifier ispumped with pump light which is transmitted in the active optical waveguide (4) at several wavelengths.
 15. Use of an optical amplifieraccording to claim 2 as a broad band optical light source with a shortcoherence length, reflection devices (20, 21), which Just transmit asmall portion of the optical energy, being provided at the end of thewave guide (4).
 16. An optical amplifier according to claim 13, whereinthe pump light of lowest order wavelength is LP₀₁.
 17. An opticalamplifier according to claim 3, wherein the wave guide (4) is made as astep index wave guide with an active doping of Nd³⁺ in a region definedby the following relationships:

    a.sub.1 /a is in the range of 0.6 to 1.3

    a.sub.2 /a is in the range of 0.85 to 4.0.